1) Field of the Invention
This invention relates to high voltage switching apparatus, and in particular, but not exclusively to a switching apparatus for providing pulses to a pulse switched load.
2) Description of Related Art
Our UK patent applications GB9929074.5 and GB9928049.7 describe a pulsed switching apparatus for an eht load such as a magnetron. A stack of FET switch modules are arranged in an oil filled chamber surrounded by four capacitors which are mounted within a plastics housing. The switch stack receives an eht supply, typically at about −55 kv and delivers a series of eht pulses to the magnetron. The switch also includes various control circuitry which operates at lt voltages. This circuitry controls functions such as triggering of the FET switches.
In these devices, when the load is switched, it is desirable for the voltage to rise smoothly to the operating voltage as shown in FIG. 1. In practice, however, there is a tendency to a voltage overshoot as shown in FIG. 2. This is because the load is not a purely resistive load but is of a non-linear nature and includes a capacitive component.
A known way of correcting for this voltage overshoot is to include a further capacitance in parallel with the load as shown in FIG. 3. In that figure, the load is shown as RL, the switch as S and the added capacitance as Cd. The problem with this approach is that the added capacitance is large, typically in the order of 200 pF, expensive and very large in size. It is therefore desirable to find an alternative way of compensating for voltage overshoot.